fix(engine): drop card-face border to remove gray-corner artifact

Player feedback after the 2-colour revert: "I do not like the
grey corners on the cards." The visible artifact was anti-
aliasing physics — the 1 px suit-coloured stroke (red for
hearts/diamonds, near-white for clubs/spades) faded through
gray pixels into the dark play surface at each rounded corner,
producing a visible "gray sliver" at the four arcs of every
card.

Fix: drop the stroke entirely. The card body fill defines the
shape against the play surface; the 5-unit brightness gap
between `#1a1a1a` body and `#151515` surface is enough to read
as a card edge without an explicit stroke. Anti-aliasing on a
fill-only rounded rect blends `#1a1a1a → #151515` over a few
pixels — barely perceptible compared to the
`stroke → transparent` gradient that produced the artifact.

### Changes

- `card_face_svg.rs`: removed `stroke="{colour}" stroke-width="2"`
  from the card body rect. Reverted the 1 px stroke inset back
  to `(x=0, y=0, width=256, height=384)` since there's no
  longer a stroke to keep inside the pixmap. Module-level
  comment updated to document the reasoning.
- `design-system.md` § Game Cards line 225 updated: "Border:
  1px solid in suit color" → "Border: none." with the
  artifact rationale recorded as audit trail.
- `card_face_svg_pin.rs` rebaselined: all 52 face hashes drift
  (every card's perimeter pixels changed); 5 back hashes
  unchanged.

Workspace clippy + cargo test --workspace clean. 1191 passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/tests/card_face_svg_pin.rs

@@ -24,58 +24,58 @@ use solitaire_engine::assets::card_face_svg::{
 use solitaire_engine::assets::rasterize_svg;
 
 const EXPECTED: &[(&str, u64)] = &[
-    ("face_AC", 0xecfed9881dab58cc),
-    ("face_2C", 0x9d226854e375a071),
-    ("face_3C", 0x57635bd3396b1c6f),
-    ("face_4C", 0x3db4ebca46202411),
-    ("face_5C", 0x6aaaa97f8d64d141),
-    ("face_6C", 0x45ab0e3692f5086f),
-    ("face_7C", 0xb6c6d47a9c41c042),
-    ("face_8C", 0x95e467bebfe1f43f),
-    ("face_9C", 0x78c67114e728f726),
-    ("face_10C", 0x59ea22af2a519731),
-    ("face_JC", 0x0757d9cae053863e),
-    ("face_QC", 0xc3de9e10c2e8819e),
-    ("face_KC", 0xefd2e9dd4c6f734f),
-    ("face_AD", 0x95e2954416f7051d),
-    ("face_2D", 0xfa494e129a7d130b),
-    ("face_3D", 0x493f32ac1b4f1365),
-    ("face_4D", 0x1303407818e3896d),
-    ("face_5D", 0x3c68bc01d5661c9b),
-    ("face_6D", 0x4ae0872812942c95),
-    ("face_7D", 0xf4a040f288b53a3d),
-    ("face_8D", 0xb5964ffbcc1834c0),
-    ("face_9D", 0xfc2b244f9e6c987c),
-    ("face_10D", 0xc9648dfd2f74e387),
-    ("face_JD", 0x055c9e4b1f56b2b4),
-    ("face_QD", 0x05d0d7e3be132b36),
-    ("face_KD", 0x540753328025961e),
-    ("face_AH", 0x8ac76ac84674dae6),
-    ("face_2H", 0xf20c188bc5cf1008),
-    ("face_3H", 0xc604901c0da15c0e),
-    ("face_4H", 0x371c115d9292fa56),
-    ("face_5H", 0x5cabef7840c6e378),
-    ("face_6H", 0x48948872acab515e),
-    ("face_7H", 0x49e96e37591f8c86),
-    ("face_8H", 0xe30b740fd0f3575b),
-    ("face_9H", 0x4067a838eeff2ea7),
-    ("face_10H", 0xd9e9913fa5d9b974),
-    ("face_JH", 0xe4344bff58d04e7f),
-    ("face_QH", 0xf33df3f193827f25),
-    ("face_KH", 0x8ada887b665fa3fd),
-    ("face_AS", 0x586d5587ad518f46),
-    ("face_2S", 0xbc0deb204e690d57),
-    ("face_3S", 0xac04b5df8741d889),
-    ("face_4S", 0x6a2ebcdb517b7ab7),
-    ("face_5S", 0x9868f72763bbdae7),
-    ("face_6S", 0x9a4c6842e0cbc489),
-    ("face_7S", 0x15d17732dadf2ec0),
-    ("face_8S", 0xb581df40dace0e59),
-    ("face_9S", 0xce92a55ddcc6b4fc),
-    ("face_10S", 0x1d92560a36938e97),
-    ("face_JS", 0xd339b7a54139f9d4),
-    ("face_QS", 0x59eae032af251c74),
-    ("face_KS", 0x901e0d1ace6ff6a9),
+    ("face_AC", 0x615e0ae429f479d5),
+    ("face_2C", 0xb882a01af5338788),
+    ("face_3C", 0x2ca39e5044b20caa),
+    ("face_4C", 0xf1439571d0877e48),
+    ("face_5C", 0x30254c808947a0e8),
+    ("face_6C", 0xb23c1bec933a3a2a),
+    ("face_7C", 0x250d5dca1cd7f72f),
+    ("face_8C", 0xe663eb85d871e71a),
+    ("face_9C", 0xd60724ce062f1c3b),
+    ("face_10C", 0x271554347971c038),
+    ("face_JC", 0xe9e6d9105d4fdd73),
+    ("face_QC", 0x3e9c1094ce524373),
+    ("face_KC", 0x8bdd75d0c9b2f23a),
+    ("face_AD", 0x452313abfc8ae82b),
+    ("face_2D", 0x5e7bfe77ab0b28a5),
+    ("face_3D", 0x327a1e905aea8beb),
+    ("face_4D", 0x86a4d1f243c60687),
+    ("face_5D", 0x0e806a2b7350efc5),
+    ("face_6D", 0x18150445cdba5fcb),
+    ("face_7D", 0x25891a7b57050f7f),
+    ("face_8D", 0x17096711946662be),
+    ("face_9D", 0x1015f68680fc63b6),
+    ("face_10D", 0x828bb4a68d291b3d),
+    ("face_JD", 0x07b88b412f1357de),
+    ("face_QD", 0x79d83db7e08d6338),
+    ("face_KD", 0x72e59e0b36af3ac0),
+    ("face_AH", 0xe8591acb1b311f68),
+    ("face_2H", 0x0ecbabd6851a6e06),
+    ("face_3H", 0x26f618607d72fb28),
+    ("face_4H", 0xd678c1b9fe409d54),
+    ("face_5H", 0xc6d600ca7b935aa6),
+    ("face_6H", 0xcccc4f21cdf2c708),
+    ("face_7H", 0x5c73195762121eec),
+    ("face_8H", 0xc8e5adc5a1878635),
+    ("face_9H", 0xfc1a1962879e3fed),
+    ("face_10H", 0x0e2bcd01a63bb11e),
+    ("face_JH", 0x9b18ac201230d355),
+    ("face_QH", 0xc98e562402c11083),
+    ("face_KH", 0x36a1eca09821b25b),
+    ("face_AS", 0x0a62cb01f3c6a27b),
+    ("face_2S", 0xa42a55c0df68c582),
+    ("face_3S", 0xf789fa97b5e9fff0),
+    ("face_4S", 0x7ffe2bc702a019c2),
+    ("face_5S", 0xe38731d462109022),
+    ("face_6S", 0xf7e7570631786c70),
+    ("face_7S", 0x4b70162e6a977a91),
+    ("face_8S", 0xe45989c24d21fda0),
+    ("face_9S", 0x5ae9856cd14f6e65),
+    ("face_10S", 0x77ff64bff391d7f2),
+    ("face_JS", 0xb27564785cb9d07d),
+    ("face_QS", 0x072a9bfccdbc367d),
+    ("face_KS", 0xbd1464c949ffa380),
     ("back_0", 0xfd1742ebe330481a),
     ("back_1", 0x446fdc0a3c83a03a),
     ("back_2", 0xcf188fdec9f5819a),